1. Field of the Invention
The present invention relates an image forming apparatus such as a copying machine, printer, facsimile machine and the like.
2. Related Background Art
In image forming apparatuses such as copying machines, printers, facsimile machine and the like, it is known to arrange a plurality of sheet supply cassettes within the apparatus in an up-and-down direction so that sheet s in the cassettes can selectively be supplied to an image forming station. In such an image forming apparatus, since the plurality of sheet supply cassettes (of front loading type which can be mounted to and dismounted from the apparatus from a front side of the apparatus) containing the sheets having different sizes are arranged within the apparatus, it is possible to reduce the time required for exchanging the sheets. Further, since the plurality of cassettes are arranged within the apparatus in the up-and-down direction, it is possible to reduce the installation space for the image forming apparatus.
In FIG. 6 showing an example of a conventional color image forming apparatus (copying machine) of the above type, a photosensitive drum 2, a transfer drum 3 and the like are arranged within the copying machine 1 at an upper part thereof, and a plurality of sheet supply cassettes 5, 6, 7 and 9 containing sheets having different sizes are arranged within the copying machine 1 at a lower part thereof.
The photosensitive drum 2 is rotated in a clockwise direction. Around the photosensitive drum 2, there are arranged a primary charger 10 for uniformly charging the rotating photosensitive drum 2; a magenta developing device 11, a cyan developing device 12, a yellow developing device 13 and a black developing device 15, which developing devices serve to develop an electrostatic latent image formed on the photosensitive drum 2; a cleaning blade 16 for scraping the residual toner remaining on the photosensitive drum 2; a collection roller 17 for collecting the toner scraped by the cleaning blade 16; and a potential sensor 19 for detecting a biasing condition of the photosensitive drum 2 to control the bias applied to the primary charger 10 and the developing devices 11, 12, 13 and 15.
An original 21 rested on a glass platen 20 of the copying machine 1 is illuminated by a illuminating lamp 22 from the below. An illuminated image on the original is incident to a lens 27 via reflection mirrors 23, 25 and 26 which are shifted at predetermined speeds, thereby focusing an light image on a taking element 29. The light image is photoelectrically converted into a signal by the taking element 29, which signal is in turn inputted to a laser scanner 31 through an image processing portion 30. Laser light L emitted from the laser scanner 31 is sent to an image writing position a on the photosensitive drum 2 via a reflection mirror 32.
In this way, the image of the original 21 is successively written on the photosensitive drum 2 which has been uniformly charged by the primary charger 10 and which is being rotated in the clockwise direction, thereby forming the electrostatic latent image on the photosensitive drum. Then, the electrostatic latent image is visualized by color toners in the developing devices 11, 12, 13 and 15.
The transfer drum 3 comprises a cylindrically shaped plastic film and is rotated in an anti-clockwise direction. Around the transfer drum 3, at predetermined positions, there are arranged an adsorption charger brush 33 and an adsorption roller 35, which serve to adhere the sheet fed from the sheet supply cassette 5, 6, 7 or 9 to the transfer drum 3; a transfer brush 36 for bias-transferring the toner image on the photosensitive drum 2 onto the sheet adhered to the transfer drum 3; a charge removing device 37 for removing the charge from the surface of the sheet, and a separating pawl or claw 39 for separating the sheet from the transfer drum 3.
The sheet supplied from the sheet supply cassette 5, 6, 7 or 9 is sent between the transfer drum 3 rotated in the anti-clockwise direction and the adsorption roller 35 and is adhered to the transfer drum 3 by the adsorption charger brush 33 and the adsorption roller 35. Then, the toner image on the photosensitive drum 2 is bias-transferred onto the sheet adhered to the transfer drum 3 color by color by the transfer brush (image forming portion) 36.
The charge on the sheet on which the toner image was transferred is removed by the charge removing device 37, and then, the sheet is separated from the transfer drum 3 by the separating pawl 39. The separated sheet is fed to a nip between a pair of fixing rollers 41 while being attracted by a convey belt 40. The toner image is permanently fixed to the sheet with heat and pressure by the paired fixing rollers 41. Thereafter, the sheet is discharged onto a sheet discharge tray (not shown) out of the copying machine by a pair of sheet discharge rollers 42.
After the transferring operation, the residual toner remaining on the photosensitive drum 2 is scraped by the cleaning blade 16 and then is collected by the collection roller 17.
The sheet supply cassettes 5, 6, 7 and 9 are of the type capable containing the maximum size sheets and are vertically spaced apart from each other by a predetermined distance. In this case, sheet supply ends 5A, 6A, 7A and 9A of the sheet supply cassettes 5, 6, 7 and 9 are disposed at the same side (right side in FIG. 6). The sheet supply cassettes 5, 6, 7 and 9 can be inserted into and retracted from the copying machine 1 from the front side thereof in a direction perpendicular to the plane of FIG. 6 by guiding left and right guides 45A, 45B of the cassettes along left and right rails 43A, 43B of the copying machine.
The sheets contained in each sheet supply cassette 5, 6, 7 and 9 are fed out by sheet supply rollers (pick-up rollers) 46, 47, 49 and 50 rotated in an anti-clockwise direction and are sent between the transfer roller 3 and the adsorption roller 35 through sheet path 51, 52, 53 and 55 and a common sheet path 56. Incidentally, the sheet fed out from each cassette 5, 6, 7 and 9 is fed to a corresponding pair of regist rollers 57, 59, 60 and 61 where the skew-feed of the sheet is corrected. Then, each sheet is sent to a second pair of regist rollers 65 by the respective paired regist rollers 57, 59, 60 and 61 and paired feed rollers 62, 63, which are rotated at a predetermined timing, thereby finally correcting the skew-feed of the sheet. Then, the sheet is sent between the transfer roller 3 and the adsorption roller 35 by the second pair of regist rollers 65 which are rotated at a predetermined timing.
The sheet fed by the paired regist rollers 65 is adhered to the transfer drum 3 by the action of the adsorption charger brush 33 and the adsorption roller 35. The transfer drum 3 is rotated in such a manner that the peripheral speed of the transfer drum becomes the same as a feeding speed of the sheet. Further, the transfer drum 3 is rotated so that the sheet adhered to the transfer drum passes through a transfer station C repeatedly.
On the other hand, the photosensitive drum (image bearing member). 2 is rotated so that the magenta color toner image developed by the magenta developing device 11 is firstly transferred onto the sheet adhered to the transfer drum 3 at the transfer station C. Then, the cyan color toner image is formed on the photosensitive drum 2 by the cyan developing device 12. During the second sheet pass, the cyan color toner image is transferred onto the same sheet adhered to the transfer drum 3 at the transfer station C. Similarly, during the third sheet pass the yellow color toner image is transferred onto the same sheet, and during the fourth sheet pass the black color toner image is transferred onto the same sheet. By transferring the four color toner images to the same single sheet in a superimposed fashion, a full-color image can be obtained.
By the way, in this conventional color image forming apparatus, since the photosensitive drum 2 and the transfer drum 3 each having a large diameter are arranged within the image forming apparatus, the installation space for the plurality of the sheet supply cassettes 5, 6, 7 and 9 are greatly limited in comparison with normal mono-color image forming apparatuses. Accordingly, conventionally, when a number of sheet supply cassettes 5, 6, 7 and 9 are arranged in the limited space within the image forming apparatus as much as possible, the sheet supply roller 46 for the uppermost sheet supply cassette 5 was made small-sized or the curvature of the sheet path 51 was minimized, because the installation space for the sheet supply roller 46 and the space for forming the sheet path 51 were limited by the presence of the common sheet path 56.
By the way, the transfer drum 3 used with the color image forming apparatus must have the ability of winding the maximum size sheet; for example, when a sheet having the maximum sheet length size of 420 mm is used, a transfer drum 3 having a diameter of 160 mm is required. Further, in order to prevent the deviation of the image, the photosensitive drum 2 must have the same diameter as that of the transfer drum 3.
However, in the above-mentioned conventional color image forming apparatus, since the sheet supply roller 46 having the small diameter was used in association with the uppermost sheet supply cassette and the curvature of the sheet path 51 was reduced, when thick sheets having the greater friction or films having the greater resiliency are supplied from the uppermost sheet supply cassette, it was feared that the poor sheet supply or the sheet jam occurred. Thus, in the conventional color image forming apparatus, in order to supply the thick sheet and the film, a manual sheet supply portion (not shown) having a straight sheet path must be provided for compensating for the case where the sheet supply cassettes 6, 7 and 9 other than the uppermost one cannot be used for the thick sheet or film.
This problem occurs in not only the color image forming apparatus but also, for example, in a mono-color image forming apparatus wherein the number of sheet supply cassettes is desired to increase in a limited space within the apparatus.